The prism shown in the figure has one side silvered. The angle of the prism is $30^{\circ}$ and $\mu=\sqrt{2}$. What should be angle of incidence, if the incident ray retraces its initial path ?
(a) $50^{\circ}$
(b) $45^{\circ}$
(c) $60^{\circ}$
(d) $75^{\circ}$